Wireless receiver without agc

ABSTRACT

A wireless communication unit ( 300 ) incorporates a receiver comprising radio frequency circuitry ( 210, 220, 230, 240 ) for receiving a radio frequency signal and converting the radio frequency signal to a low frequency signal. A signal level adjustment circuit receives the low frequency signal and an analogue to digital converter ( 370 ), operably coupled to the signal level adjustment circuit receives an adjusted low frequency signal and providing a digital received signal. A signal processor ( 108 ) operably coupled to the analogue to digital converter ( 370 ) processes the digital received signal. The signal level adjustment circuit includes a low frequency amplifier ( 360 ) whose gain is arranged to be dependent upon a clip point of the analogue to digital converter ( 370 ).

FIELD OF THE INVENTION

This invention relates to radio receiver technology. The invention isapplicable to, but not limited to, removing a need for automatic gaincontrol in portable or mobile radio and/or cellular phone technology,whilst maintaining a high dynamic range.

BACKGROUND OF THE INVENTION

Wireless communications systems, for example cellular telephony orprivate mobile radio communications systems, typically provide for radiotelecommunication links to be arranged between a plurality of basetransceiver stations (BTSs) and a plurality of subscriber units, oftentermed mobile stations (MSs). The term mobile station generally includesboth hand-portable and vehicular mounted radio units.

The communication link from a BTS to an MS is generally referred to as adownlink communication channel. Conversely, the communication link froman MS to a BTS is generally referred to as an up-link communicationchannel.

Wireless communication systems are distinguished over fixedcommunications systems, such as the public switched telephone networks(PSTN), principally in that mobile stations move between serviceproviders (and/or different BTS) and in doing so encounter varying radiopropagation environments.

In a wireless communication system, each BTS has associated with it aparticular geographical coverage area (or cell). The coverage areadefines a particular range over which the BTS can maintain acceptablecommunications with MSs operating in its serving cell. Often these cellscombine to produce an expanded system coverage area, with theinfrastructure supporting respective cells interconnected viacentralised switching equipment.

The coverage area is typically determined by a receiver's ability toreceive and decode very low-level signals from the transmitting unit.The range of signal levels that a receiver must be able to receive istermed the ‘dynamic range’ of a radio receiver.

For present day hand-portable or mobile receivers, such as those used inportable two-way radios and cellular phones, a larger dynamic rangeperformance equates to less dropped calls and less cell-to-cellhandovers. This further leads to a reduction in system overheadtransmissions as well as a general improvement to the system's qualityof service. Therefore, it is generally a desirable aim of a receiverdesigner to increase/improve a receiver's dynamic range.

However, a trade-off exists between dynamic range and currentconsumption in such receivers. Such a trade-off is particularlyimportant iN the field of portable radio designs, where battery life andtherefore power consumption is of critical importance. To obtain ahigher dynamic range, the active stages of a receiver's radio frequency(RF) components must run at higher DC currents, thus lowering batterylife during standby. Running at higher DC currents consequently causes areduction in battery life.

One option to compensate for a reduction in battery life, in order toincrease a dynamic range, would be to increase the battery capacity.However, such an option is rarely considered as viable in the portablecellular/radio field, as this would require an increase in both the sizeand the weight of the battery.

In summary, reducing standby time or increasing battery capacity areboth highly undesirable for portable products, where low size and weightand long standby times are demanded in order for a product to becompetitive in this market.

A radio receiver is often defined in terms of ‘front-end’ and ‘back-end’characteristics. The front-end of a receiver encompasses all of the RFcircuitry whereas the ‘back-end’ encompasses all of the base-bandprocessing circuitry.

In the field of this invention, it is known that most receivers haveautomatic gain control (AGC), which controls the gain of the finalstages of the receiver. Such AGC operation needs to be wide-band whensome portion of the AGC circuitry is operational at RF frequencies, ornarrow-band if the AGC circuitry is only operational at intermediate orbaseband frequencies.

When used in receiver circuits, amplifiers will often encounter a verywide range of signal levels; typically they need to operate (namely beable to receive and recover signals) over a 120-dB range. To preventoverloading of these active components, the receiver's gain must bereduced as the signal strength increases. This is usually achievedautomatically by changing the bias point of the RF transistor. However,in addition to the change in gain, the bias changes may also alter theinput and output impedances of the receiver's front-end and thereby maymake the amplifier operation more nonlinear.

Overall amplifier gain is generally dictated by the performance of twoamplifier parameters. The first component is the power gain of thetransistor itself and the second component is the loss of gain due toinput and output mismatching.

A poor design might then start out with a high-gain mismatched stage.When the bias is changed in order to reduce the gain of the transistor,the input and output impedance matching needs to change in order toprovide an improved transfer of power.

A better design would start out with ideal matching when highest gain isrequired and then benefit from both the mismatch and the transistor gainloss as bias is changed. Such a design would likely requireneutralization at the highest gain setting.

It is known that the gain of a transistor is typically changed in one oftwo ways. A first method is to change the collector current or thecollector-to-emitter voltage. Any current changes either above or belowthe optimum value result in a loss of gain. At lower current levels, thevoltage has little effect on the gain, whilst at the higher currentlevels the collector voltage has a more noticeable effect. The twomethods of automatic gain control will then depend on whether thecollector current is either increased (forward control) or decreased(reverse control) from the optimum value.

The simplest method of gain control is obtained by reducing thecollector current. This will simultaneously decrease the current gainand raise the input impedance. Both effects will decrease the powergain, and the increase in input impedance will result in a furthermismatch loss.

In summary, it is known that the provision of stable, AGC circuits inradio products is problematic, complex and typically requires asubstantial number of hardware (RF) components. In particular, AGCdesigns are critical to ensure stability of the RF operation, and toachieve rapid receiver operation, often-termed fast ‘receiver attacktimes’. The costly hardware needs to be controlled from the base-bandprocessing circuitry, in order to ensure accurate performance, whichadds to the complexity of the AGC solution.

A need therefore exists for a high dynamic range radio receiver,particularly for linear technology such as that adopted for TETRA,wherein the above-mentioned disadvantages may be alleviated.

STATEMENT OF INVENTION

In accordance with a preferred embodiment of the present invention,there is provided a wireless communication unit incorporating areceiver, as claimed in claim 1. A method as claimed in claim 16 is alsoprovided. Further aspects of the invention are as claimed in thedependent claims.

In summary, the preferred embodiment of the present invention proposes areceiver arrangement that uses a dynamic compressor function, inconjunction with a modified amplifier, to provide a high dynamic rangereceiver that precludes the need to use AGC circuits or components.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the present invention will now be described,with reference to the accompanying drawings, in which:

FIG. 1 shows a block diagram of a radio communication unit that can beadapted to support the various inventive concepts of the preferredembodiment of the present invention, with regard to receiver dynamicrange;

FIG. 2 shows a more detailed block-diagram representation of a prior artreceiver arrangement employing automatic gain control to improve areceiver's dynamic range; and

FIG. 3 shows a more detailed block-diagram representation of a receiverarrangement and method, adapted to incorporate the inventive concepts ofthe present invention, negating the need for an AGC circuit.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring now to FIG. 1, a block diagram of a subscriber unit/mobilestation (MS) 100, adapted to support the inventive concepts of thepreferred embodiments of the present invention, is shown.

The MS 100 contains an antenna 102 preferably coupled to a duplex filteror circulator or antenna switch 104 that provides isolation betweenreceive and transmit chains within MS 100.

The receiver chain includes scanning receiver front-end circuitry 106(effectively providing reception, filtering and intermediate orbase-band frequency conversion). The scanning front-end circuit 106 isserially coupled to a signal processing function (processor, generallyrealised by a DSP) 108 via a baseband (back-end) processing circuit 107.The preferred arrangement of the receiver chain is described in moredetail with regard to FIG. 2.

In accordance with a preferred embodiment of the present invention, thereceiver chain 110 has been substantially adapted as described in moredetail with regard to FIG. 3. In particular, the signal processingfunction 108, in conjunction with the scanning front-end circuit 106 andbaseband processing circuit 107, have been adapted for a receiving MS toreceive and process signals across a high dynamic range, without theneed for an automatic gain control circuit.

A controller 114 is operably coupled to the scanning front-end circuitry106 so that the receiver can calculate receive bit-error-rate (BER) orframe-error-rate (FER) or similar link-quality measurement data fromrecovered information via a received signal strength indication (RSSI)112 function. The RSSI 112 function is operably coupled to the scanningfront-end circuit 106. The memory device 116 stores a wide array ofMS-specific data, for example decoding/encoding functions, dynamic rangeperformance information based on ‘on-channel’ and ‘off-channel’ signalmeasurements obtained from, say, the RSSI function 112.

A timer 118 is operably coupled to the controller 114 to control thetiming of operations, namely the transmission or reception oftime-dependent signals, within the MS 100.

As known in the art, received signals that are processed by the signalprocessing function are typically input to an output device, such as aspeaker or visual display unit (VDU).

For completeness, the transmit chain essentially includes an inputdevice 120, such as a microphone, coupled in series through a processor108, transmitter/modulation circuitry 122 and a power amplifier 124. Theprocessor 108 transmitter/modulation circuitry 122 and the poweramplifier 124 are operationally responsive to the controller, with anoutput from the power amplifier coupled to the duplex filter orcirculator or antenna switch 104, as known in the art.

It is within the contemplation of the invention that the microprocessor108 and controller arrangement 114 may be combined into one element,operably connected or separate functions of each interconnected in areasonable manner as known in the art.

As known in the art, substantially the same circuitry of FIG. 1, (andFIG. 3) is contained within a serving communication unit/BTS, albeitwith some circuits being adapted in terms of processing power, higheroutput power for transmission to multiple remote communication units,multiple signal paths for receiving signals from multiple remotecommunication units, etc. As such, when the circuitry in FIG. 1 isapplied to a serving Base Transceiver Station (BTS), the serving BTS hasan adapted receiver/processor arrangement to avoid the need for an AGCcircuit according to the preferred embodiment of the present invention.In particular, if multiple receiver paths are used, each path no longerrequires an AGC circuit or one or more AGC loops, thereby simplifyingsignificantly the design of the BTS receiver.

In either the MS 100, or the BTS case, the signal processor function 108in the transmit chain may be implemented as distinct from the processorin the receive chain. Alternatively, a single processor 108 may be usedto implement the processing of both transmit and receive signals, asshown in FIG. 1.

Of course, the various components within the MS 100 or a similar BTS canbe realised in discrete or integrated component form. Furthermore, it iswithin the contemplation of the invention that the MS 100 may be anyradio receiver device, such as a portable or mobile PMR radio, a mobilephone, a personal digital assistant, a number of wirelesslyinter-connectable laptop computers.

Referring now to FIG. 2, a more detailed block diagram of a prior artconventional receiver arrangement is shown, incorporating an AGC circuitto provide a receiver with high dynamic range. The receiver arrangementis suitable for receiving linear modulation signals, such as aπ/4-Differential Quadrature Phase Shift Keyed (DQPSK) modulation signalin compliance with the TETRA standard.

Linear modulation technologies utilise distinct phase and/or amplitudevalues of signals to convey data, as known to those skilled in the art.Such data is conveyed in a format where a number of phase/amplitudepositions equate to a number of bits, for example a Quadrature PhaseShift Keyed (QPSK) signal comprises four phase-shifted constellationpositions, each position definable by two bits—‘0, 0’, ‘1, 0’, ‘0, 1’and ‘1, 1’. The term “linear” is often used to describe such digitalmodulation techniques, as the received signal needs to remain in the“linear” regions of the circuits and components used to transmit orrecover signals.

Consequently, linear modulation receivers do not use limiters, which areknown to distort the integrity of the received signal. The integrity ofreceived signal is judged according to the magnitude of vector errors,whereby a particular received collection of bits (2-bits in the QPSKcase), is assessed in a vector context to see how far it is from theideal case. The vector is judged in a 2-dimensional context, taking intoaccount the amplitude and phase of the received signal.

In FIG. 2, signals enter the receiver chain via the antenna 202 andarrive at the input of a RF pre-selector band-pass filter (BPF) 210 viaan adjustable RF attenuator 205. The RF filter 210 is not capable ofbeing a narrowband (on-channel) device at such RF frequencies, butserves to reject out-of-band signals. The filtered signal is input to alow-noise pre-amplifier (LNA) 220. The LNA 220 typically has anadjustable DC current setting, which determines the dynamic range of theLNA 220.

The amplified signal leaves the LNA 220 and is input to a second RFband-pass filter 230 via a second RF attenuator 225. The second RFfilter 230 further filters out-of-band interfering signals, for exampleimage signals to which the mixer is sensitive and which would theninterfere with desired signals.

The RF filtered signal is then typically input to a mixer 240, whichuses a local oscillator signal to down-convert the frequency of thereceived signal to an intermediate frequency (IF) (or direct conversionto a baseband frequency). If a heterodyne receiver architecture is used(as shown), the intermediate frequency signal is further filtered by theIF filter 250 and input to the back-end circuit 207, where the IF signalis amplified by IF amplifier 260.

The amplified IF signal is then attenuated by IF/baseband attenuator 265and the IF/baseband signal is then converted from an analogue signal toa digital signal, for example using sigma-delta conversion technology,as known to those skilled in the art, in A/D converter 270.

A/D converters are known to have a limited dynamic range of the order of25 dB, which leads to the requirement of substantial AGC circuitry toensure the input signal to the A/D converter is at an appropriate signallevel. Furthermore, the A/D converter 270 includes anti-aliasingfilter(s) and decimation filter(s) that assist the band-pass filteringoperation of the receiver. The digital signal output from A/D converter270 is then demodulated and processed using signal processor 208.

In operation, the signal processor 208 and/or A/D converter 270 performthe control of the dynamic range of the receiver circuit, i.e. the AGCfunction, by adjusting any number of components such as RF attenuators205, 225, LNA 220, mixer 240, IF/baseband amplifier 260 or IF/basebandattenuator 265. Clearly, the more components and/or circuits used in theAGC function, the greater the number of AGC ‘loops’ and the greater thechance of instability. In a typical receiver, attenuator blocks areimplemented using switchable attenuators or linear AGC functionality,which are accurate and easy to control, but are costly and require alarge amount of space.

If a very strong signal, or a very weak signal, appears at the input ofthe LNA 220, the signal processor 208 and/or A/D converter 270 adjustthe performance of one or more of the above components to ensure thatthe subsequent processing of the received signal does not distort oraffect its integrity. In this manner, the signal processor 208 and/orA/D converter 270, ensures the receiver operates over a wide dynamicrange. In particular, the signal processor 208 and/or A/D converter 270prevent the desired or an interfering signal from over-loading thereceiver, thereby avoiding loss of gain and/or distortion of the wantedsignal.

Referring now to FIG. 3, a more detailed block-diagram representation ofa receiver arrangement and method is shown, in accordance with thepreferred embodiments of the present invention. Of particular note isthe lack of an AGC circuit and AGC components when compared to prior artreceiver arrangements, such as that described with reference to FIG. 2.

In the same manner to known radio frequency receivers, signals enter thereceiver chain via the antenna 102 and arrive at the input of a RFpre-selector band-pass filter (BPF) 210. The RF filter 210 output signalis input to a low-noise pre-amplifier (LNA) 220. The LNA 220 may includean adjustable DC current setting, which would help to determine thedynamic range of the LNA 220. Alternatively, as a consequence of theinventive concepts herein described, the LNA 220 controllingvoltage/current values may be fixed.

The amplified signal leaves the LNA 220 and is input to a second RFband-pass filter 230. The second RF filter 230 further filtersout-of-band interfering signals, for example image signals generatedduring the LNA amplification process.

The RF filtered signal is again typically input to a mixer 240, whichuses a local oscillator signal to down-convert the frequency of thereceived signal to an intermediate frequency (IF) (or direct conversionto a baseband frequency). If a heterodyne receiver architecture is used(as shown), the intermediate frequency signal is further filtered by theIF filter 250 and input to the back-end circuit 107.

In the preferred embodiment of the present invention, the back-endcircuit 107 includes a modified IF/baseband amplifier (XAmp) 360.

The preferred equation isGXAmp=GIFamp+|LCP−ADCP|+ADCPM  [1]where:

-   -   GXAmp=Gain of XAmp in dB;    -   GIFamp=Gain of IF Amplifier 260 (of FIG. 2);    -   LCP=Dynamic compressor clip point;    -   ADCP=A/D clip point; and    -   ADCPM=additional Margin to A/D clip point.

As shown above, the modified IF/baseband amplifier (XAmp) 360 amplifiesthe IF filtered signal with a higher gain than with a typical knownprior art IF amplifier. The output of the XAmp 360 is connected to theinput of the dynamic compressor function 362.

In the preferred embodiment of the present invention, the dynamiccompressor function 362 may take the form of a simple ‘limiter’ stage.Preferably, band-pass filtering is also used within the dynamiccompressor function 362 to remove any signal harmonics.

The output of the dynamic compressor function 362 is input to a modifiedIF/baseband attenuator function 365. The modified IF/baseband attenuatorfunction 365 is connected to the A/D, for example digital backend SigmaDelta converter 370, and attenuates the received signal level to asufficient level to prevent it from clipping in the A/D.

It is noteworthy that the modified IF/baseband attenuator function 365is fixed, whose value is calculated as:Attenuation [dB]=|LCP−ADCP|+ADCPM  [2]

The digital signal output from A/D converter 270 is then demodulated andprocessed using microprocessor 108.

In accordance with the preferred embodiment of the present invention,the introduction of a higher and specific gain IF/baseband amplifier360, together with the dynamic compressor 362 has enabled the operationof the A/D converter 270 and/or microprocessor 108 to be adapted suchthat they no longer need to support an AGC function.

The preferred embodiment of the present invention is particularlyapplicable to TETRA π/4-DQPSK modulated signals. However, it is withinthe contemplation of the invention, that the inventive concepts hereindescribed will also work with many other linear modulation schemes.

Furthermore, the inventor of the present invention has found that thereis negligible impact in vector-error magnitude when implementing thedynamic compressor together with the modified XAmp 360. In addition, theuse of the dynamic compressor avoids the possibility of a high receivedsignal exceeding the clip point of the sigma-delta A/D converter 370.

Notwithstanding the above removal of all AGC circuits and components inthe preferred embodiment of the invention, it is within thecontemplation of the invention that minimal AGC may be employed. Oneexample would be in providing AGC performance of the order of 10 db or20 dB, which may be available as a standard offering in integrateddigital backend ICs. Although AGC is no longer required, such levels aretypically provided for in sigma-delta A/D converters and may thereforebe of some use in fine-tuning received signal levels.

In implementing the receiver arrangement of FIG. 3, an operationaldynamic range of 130 dB or more can be achieved without any AGC or, ifused, with only 10 dB to 20 dB of AGC in the A/D (digital backend SigmaDelta) converter 370. With this approach a TETRA bit error rate of zerofailures can be achieved for signals up to +16 dBm or more whilstmeeting all receiver specifications. Advantageously, the above receiverarrangement eliminates the many disadvantages of conventional linearreceivers requiring AGC circuits, and other (non-linear) receivers thatrequire a more complex wide-band AGC operation, as previously described.

It is further within the contemplation of the invention that alternativereceiver architectures can be used that would also benefit from theinventive concepts described herein, such as direct conversionreceivers, superheterodyne receivers etc.

Thus, a wireless communication unit incorporating a receiver has beendescribed. The receiver includes radio frequency circuitry for receivinga radio frequency signal and converting said radio frequency signal to alow frequency signal. A signal level adjustment circuit receives saidlow frequency signal; and an analogue to digital converter, operablycoupled to said signal level adjustment circuit, receives an adjustedlow frequency signal and provides a digital received signal. A signalprocessor is operably coupled to the analogue to digital converter forprocessing said digital received signal. The signal level adjustmentcircuit includes a low (or intermediate) frequency amplifier whose gainis arranged to be dependent upon a clip point of said analogue todigital converter.

In this manner, AGC circuits and components are not required,particularly in relation to a digital receiver compliant with the TETRAstandard.

Whilst specific, and preferred, implementations of the present inventionare described above, it is clear that one skilled in the art couldreadily apply variations and modifications of such inventive concepts.

1. A wireless communication unit incorporating a receiver, the receivercomprising: radio frequency circuitry for receiving a radio frequencysignal and converting said radio frequency signal to a low frequencysignal; a signal level adjustment circuit for receiving said lowfrequency signal; an analogue to digital converter, operably coupled tosaid signal level adjustment circuit for receiving an adjusted lowfrequency signal and providing a digital received signal; and a signalprocessor operably coupled to the analogue to digital converter forprocessing said digital received signal; wherein said signal leveladjustment circuit includes a low frequency amplifier whose gain isarranged to be dependent upon a clip point of said analogue to digitalconverter.
 2. The wireless communication unit according to claim 1,wherein the signal level adjustment circuit is further by comprises adynamic compressor function, operably coupled to said low frequencyamplifier for limiting a signal output from said low frequencyamplifier.
 3. The wireless communication unit according to claim 2,wherein the gain of said low frequency amplifier is arranged to bedependent upon a clip point of said dynamic compressor function.
 4. Thewireless communication unit according to claim 3, wherein the gain ofsaid low frequency amplifier is arranged to be dependent upon the clippoint of said dynamic compressor function subtracted by the clip pointof said analogue to digital converter.
 5. The wireless communicationunit according to any of claims 2, wherein said signal level adjustmentcircuit further comprises a fixed attenuator operably coupled to saiddynamic compressor function to attenuate a received signal output fromsaid dynamic compressor function to below a clip point threshold of saidanalogue to digital converters.
 6. The wireless communication unitaccording to claim 5, wherein said fixed attenuator is arranged to bedependent upon a clip point of said analogue to digital converter. 7.The wireless communication unit according to claim 5, wherein said fixedattenuator is arranged to be dependent upon a clip point of said dynamiccompressor function.
 8. The wireless communication unit according toclaim 17, wherein said fixed attenuator is arranged to be dependent uponthe clip point of said dynamic compressor function subtracted by theclip point of said analogue to digital converter.
 9. The wirelesscommunication unit according to claim 1, wherein said low frequencycomponents are at an intermediate or baseband frequency.
 10. Thewireless communication unit according to claim 1, wherein said receiverhas a high dynamic range, for example in excess of 100 dB.
 11. Thewireless communication unit according to claim 1, wherein said signallevel adjustment circuit negates a need for an automatic gain controlcircuit.
 12. The wireless communication unit according to claim 1,wherein the wireless communication unit is a subscriber unit or a basetransceiver station operating in a wireless communication system. 13.The wireless communication unit according to claim 12 wherein thesubscriber unit is one of a portable or mobile PMR radio, a mobilephone, a personal digital assistant, a wireless capable laptop computer.14. The wireless communication unit according to claim 1, wherein thereceived signal is a digitally modulated signal.
 15. The wirelesscommunication unit according to claim 14, wherein the receiver is alinear receiver for receiving said digitally modulated signal.
 16. Amethod of signal reception for a wireless communication unit, the methodcomprising: receiving a radio frequency signal and converting said radiofrequency signal to a low frequency signal; adjusting the signal levelof said low frequency signal; analogue to digital converting the signalwith an analogue to digital converter after said signal level adjustmentstep, thereby providing a digital received signal; and signal processingof the said digital received signal; wherein said signal leveladjustment circuit includes low frequency amplification with a gainarranged to be dependent upon a clip point of said analogue to digitalconverter.
 17. The wireless communication unit according to claim 6,wherein said fixed attenuator is arranged to be dependent upon a clippoint of said dynamic compressor function.